Many types of machines are assembled using such common components as motors, drive shafts, bearings and gears. Mobile machines (machines capable of self-propelled travel) including particular types thereof are no exception. Mobile machines are used for earthmoving, excavating and, in the case of open-pit mining, to remove overburden and expose the valuable mineral or ore beneath such overburden. A dragline, a particular type of mobile machine, is usually used for open-pit mining.
In a smaller machine, "unitary" components are used in assembly. As used in this specification, a unitary component is formed as a single piece, i.e., a complete shaft, gear or the like.
An example of how unitary gear components are used to make a crane swing drive appears in U.S. Pat. No. 4,478,340 (Delago). The bull gear or circle gear shown in FIG. 3 of the Delago patent is made as a single piece. The "upper works" of the crane is caused to rotate on the truck-like chassis by a pinion gear engaged with the circle gear.
However, as machine size increases, it becomes progressively more impractical to make some components as unitary components. In a very large machine, such components would be all-but-impossible to make--machine tools of suitable size either do not exist or are not readily available. And in any event, such a component would be unwieldly to handle.
A specific example involves the swing drive of a large digging machine. If the machine is relatively large, the swing drive will probably have drive components which are "segmented," i.e., made in several pieces. The components shown in FIG. 2 of U.S. Pat. No. 3,888,357 (Bauer et al.) and in FIGS. 3 and 4 of U.S. Pat. No. 4,622,860 (Cametti et al.) are examples.
In the manufacture of large draglines such as shown in U.S. Pat. No. 5,320,431 (Kallenberger), it has been the practice to provide the swing "rack" or bull gear as a number of arc-like gear segments. Such segments are placed end-to-end to form the gear circle and the segments are then mounted on the segment support structure or "tub" as it is often called. Mounting is by welding each segment to the support structure. (The magnitude of the gear fabrication and mounting problem will be better appreciated by understanding that the swing rack on a large dragline may have a diameter of 15-20 feet or more and weight several tons.)
A circular rail is mounted atop the assembled swing rack and bearings supporting the upper, rotating part of the dragline ride on such rail. In cross-section, such rail resembles a railroad-type rail with its outwardly-extending flanges for mounting. Rail mounting is by clearance-fit clip bolts which hold a flange-clamping clip against the rail. Such clip bolts extend through apertures in the gear segment and in the support structure.
While the gear segment mounting arrangement described above has been generally suitable, it is not without its problems. A significant disadvantage arises from the fact that draglines (as well as other types of mobile machines) are subjected to a good deal of mechanical abuse and "shock" loading.
Welds are rather brittle and the welds holding a gear segment to the support structure develop cracks and fail. The gear segment formerly secured by the failed welds must immediately be replaced or at least re-attached as the swing drive cannot be operated with even one non-functional gear segment. If worn or detached segments are to be replaced with new, the common practice is to place a new segment at a specific place formerly occupied by the worn or detached segment and then weld such new segment in place.
And the clip bolts are of little value in preventing weld cracking. Such clip bolts, being of the clearance-fit type, do not prevent slight, momentary radial shifts in position of a gear segment as a shock load is imposed on such segment. It is the slight radial shifts that cracks welds. Further, bolts are commonly used (and are intended to be used) as "tension" or clamping devices. That is, they depend for their clamping holding power upon being stretched slightly as the nut is tightened.
Broken welds and resulting machine downtime (which, with the advent of the invention, is unnecessary) translate into machine operating costs. Very clearly, a reduction in machine operating costs benefits the seller, the buyer and in the case of a dragline, those purchasing products made from the mineral being mined.
An improvement in the way a machine component such as a gear segment is mounted on a support structure would be an important advance in the art.